Laser fuseblow protection method for silicon on insulator (SOI) transistors

ABSTRACT

A method and apparatus are provided for laser fuseblow protection in transistors, such as silicon-on-insulator (SOI) transistors. The transistors are connected to a fuse. A pair of diodes are connected in series between a high supply and ground. A common connection of the series connected pair of diodes is connected to a common connection of the fuse and transistors. A charge is shunted to the high supply or ground by the pair of diodes with a first voltage a set value above the high supply and a second voltage a set value below the ground. A pair of protection diodes are provided on each side of the fuse with transistors. The transistors are either connected to one side of the fuse or to both sides of the fuse.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for laserfuseblow protection in silicon-on-insulator (SOI) transistors.

DESCRIPTION OF THE RELATED ART

Silicon-on-insulator (SOI) technology is an enhanced silicon technologycurrently being utilized to increase the performance of digital logiccircuits. Utilizing SOI technology designers can increase the speed ofdigital logic integrated circuits while reducing their overall powerconsumption. These advances in technology will lead to the developmentof more complex and faster computer integrated circuits that operatewith less power.

As shown in FIG. 1, SOI semiconductors include a thin layer of siliconplaced on top of an insulator, such as silicon dioxide (SiO₂) or glass,and a MOS transistor built on top of this structure. The SOI FETincludes a body (B), a gate (G), a drain (D) and a source (S). The mainadvantage of constructing the MOS transistor on top of an insulatorlayer is to reduce the internal capacitance of the transistor. This isaccomplished by placing the insulator oxide layer between the siliconsubstrate and the impurities required for the device to operate as atransistor. Reducing the internal capacitance of the transistorincreases its operating speed. With SOI technology faster MOStransistors can be manufactured resulting in higher performancesemiconductors for faster electronic devices.

In a conventional, bulk silicon process, the transistors are built inthe top surface of a relatively thick mass of silicon. During a laserfuseblow process, energy that is imparted to the transistors has thisthick silicon in which to dissipate this energy. SOI transistors arebuilt on the thin layer of silicon placed on top of a thicker insulatoras shown in FIG. 1. Damage to the SOI transistor devices can occur dueto the decreased ability to dissipate the resulting charge build up formthe laser fuseblow process in the thin layer of silicon.

A need exists for a mechanism for protecting SOI transistors from anexcess voltage and charge that can be built up during a laser fuseblowprocess.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a method andapparatus for laser fuseblow protection in silicon-on-insulator (SOI)transistors. Other important objects of the present invention are toprovide such a method and apparatus for laser fuseblow protection insilicon-on-insulator (SOI) transistors substantially without negativeeffect and that overcome many of the disadvantages of prior artarrangements.

In brief, a method and apparatus are provided for laser fuseblowprotection in transistors, such as silicon-on-insulator (SOI)transistors. The transistors are connected to a fuse. A pair of diodesare connected in series between a high supply and ground. A commonconnection of the series connected pair of diodes is connected to acommon connection of the fuse and transistors. A charge is shunted tothe high supply or ground by the pair of diodes with a first voltage aset value above the high supply and a second voltage a set value belowthe ground.

In accordance with features of the invention, a pair of protectiondiodes are provided on each side of the fuse with transistors. Thetransistors are either connected to one side of the fuse or to bothsides of the fuse.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIG. 1 is a cross sectional view illustrating a conventionalsilicon-on-insulator (SOI) N-channel field effect transistor (NFET);

FIG. 2 is a schematic diagram illustrating apparatus for laser fuseblowprotection in silicon-on-insulator (SOI) transistors in accordance withthe preferred embodiment; and

FIG. 3 is a schematic diagram illustrating another apparatus for laserfuseblow protection in silicon-on-insulator (SOI) transistors inaccordance with the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having reference now to the drawings, in FIG. 2, there is shownapparatus for laser fuseblow protection in silicon-on-insulator (SOI)transistors of the preferred embodiment generally designated by thereference character 200. As shown in FIG. 2, a plurality of SOItransistors 202, 204, 206 and 208 are directly connected to a fuse 210.It should be understood that the principles of the present invention arenot limited to silicon-on-insulator (SOI) transistors and can be usedwith other types of transistors, such as conventional, bulk silicontransistor devices.

In very large scale integrated (VLSI) chips, it is common to have fuses,such as fuse 210 that can be programmed for various reasons. Among thesereasons include invoking redundant elements in memory arrays forrepairing failing locations or programming identification information. Acommon method of building and programming such fuses 210 is to build athin metal wire fuse near the top surface of the chip. The fuse 210 canbe programmed by selectively deleting the thin metal using a laser (notshown). The thin wire is connected to a circuit that can sense whetherthe fuse remains or has been deleted. The laser imparts energy thatvaporizes the metal to create an open circuit. The circuit that sensesthe presence or absence of a fuse 210 must be able to withstand theenergy of the laser fuseblow process.

In accordance with features of the preferred embodiment, the problem ofprotecting the SOI transistors 202, 204, 206 and 208 connected to fuse210 to withstand the energy of the laser fuseblow process is solved by aplurality of protection diodes 212, 214, 216 and 218. Protection diodes212, 214, 216 and 218 of the preferred embodiment are designed forprotecting the SOI transistors 202, 204, 206 and 208 directly connectedto fuse 210.

There are many designs or arrangements for sensing whether a fuse 210has been blown or not. In all cases transistors are directly connectedto fuse 210 either on one side or both sides of the fuse and are proneto damage. In FIG. 2, the SOI transistors 202, 204, 206 and 208 directlyconnected to fuse 210 are protected respectively by protection diodes212, 214, 216 and 218. Protection diodes 212 and 214 and protectiondiodes 216 and 218 are connected in series between the high supply VDDand ground. As shown in FIG. 2, a cathode of diodes 212, 216 isconnected to the high supply and an anode of diodes 212, 216 isconnected respectively to a cathode of diodes 214, 218 with an anode ofdiodes 214, 218 connected to ground. A common connection of the seriesconnected protection diodes 212 and 214 is connected to the commonconnection of fuse 210 and SOI transistors 202 and 204. A commonconnection of the series connected protection diodes 216 and 218 isconnected to the common connection of fuse 210 and SOI transistors 206and 208.

By designing the protection diodes 212, 214, 216 and 218 to each powersupply on each side of the fuse 210 with transistors 202 and 204, andtransistors 206 and 208, the transistors 202, 204, 206 and 208 areprotected from the otherwise destructive voltages that could damagethem. The protection diodes 212, 214, 216 and 218 will shunt the chargeto either the ground or high supply if the voltage rises above a diodethreshold voltage or around 0.7 Volt above the high supply or dropslower than 0.7 Volt below the ground supply. Transistors are typicallydesigned to withstand these voltages that are only 0.7 Volt above orbelow typical or expected values during functional operation.

Referring to FIG. 3, there is shown another apparatus for laser fuseblowprotection in silicon-on-insulator (SOI) transistors of the preferredembodiment generally designated by the reference character 300. As shownin FIG. 3, a pair of SOI transistors 302, 304 are directly connected toone side of a fuse 310. The other side of fuse 310 is connected toground. Protecting the SOI transistors 302 and 304 connected to fuse 310to withstand the energy of the laser fuseblow process is solved by aplurality of protection diodes 312, 314, 316 and 318. Protection diodes312 and 314 and protection diodes 316 and 318 are connected in seriesbetween the high supply VDD and ground. As shown in FIG. 3, a cathode ofdiodes 312, 316 is connected to the high supply and an anode of diodes312, 316 is connected respectively to a cathode of diodes 314, 318 withan anode of diodes 314, 318 connected to ground. A common connection ofthe series connected protection diodes 312 and 314 is connected to thecommon connection of fuse 310 and SOI transistors 302 and 304. A commonconnection of the series connected protection diodes 316 and 318 isconnected to the common connection of fuse 310 and ground.

By designing the protection diodes 312, 314, 316 and 318 to each powersupply on each side of the fuse 310 with transistors 302 and 304, thetransistors 302 and 304 are protected from the otherwise destructivevoltages that could damage them. The protection diodes 312, 314, 316 and318 will shunt the charge to either the ground or high supply if thevoltage rises above a diode threshold voltage or around 0.7 Volt abovethe high supply or drops lower than 0.7 Volt below the ground supply.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

What is claimed is:
 1. A method for laser fuseblow protection intransistors directly connected to a fuse comprising the steps of:providing a pair of diodes connected in series between a high powersupply and ground; directly connecting a common connection of saidseries connected pair of diodes to a connection of the fuse andtransistors; and shunting a charge to said high power supply or groundby said pair of diodes with a first voltage a set value above said highpower supply and a second voltage a set value below said ground.
 2. Amethod for laser fuseblow protection in transistors directly connectedto a fuse as recited in claim 1 wherein the transistors includesilicon-on-insulator (SOI) transistors.
 3. A method for laser fuseblowprotection in transistors directly connected to a fuse as recited inclaim 1 further includes the steps of providing a second pair of diodesconnected in series between said high power supply and ground; anddirectly connecting a common connection of said series connected pair ofdiodes to a connection of the fuse and ground.
 4. A method for laserfuseblow protection in transistors directly connected to a fuse asrecited in claim 1 wherein the step of providing a pair of diodesconnected in series between said high power supply and ground includesthe step of connecting a cathode of a first diode of said pair of diodesto said high supply and an anode to a cathode of a second diode of saidpair of diodes and connecting an anode of said second diode to ground.5. A method for laser fuseblow protection in transistors directlyconnected to a fuse as recited in claim 1 wherein the step of shunting acharge to said high power supply or ground by said pair of diodes with afirst voltage a set value above said high power supply and a secondvoltage a set value below said ground includes the step of shunting acharge to said high power supply or ground by said pair of diodes with afirst voltage about 0.7 Volt above said high power supply and a secondvoltage about 0.7 Volt below said ground.
 6. A method for laser fuseblowprotection in transistors directly connected to a fuse comprising thesteps of: providing a first pair of diodes connected in series between ahigh power supply and ground; directly connecting a common connection ofsaid series connected first pair of diodes to a connection of the fuseand transistors; shunting a charge to said high power supply or groundby said first pair of diodes with a first voltage a set value above saidhigh power supply and a second voltage a set value below said ground;providing a second pair of diodes connected in series between a highsupply and ground; and directly connecting a common connection of saidseries connected second pair of diodes to a second side of the fuse. 7.A method for laser fuseblow protection in transistors directly connectedto a fuse as recited in claim 6 further includes the step of connectingsaid second side of the fuse to ground.
 8. A method for laser fuseblowprotection in transistors directly connected to a fuse as recited inclaim 6 further includes the step of directly connecting secondtransistors to said common connection of said series connected secondpair of diodes and said second second of the fuse.
 9. A method for laserfuseblow protection in transistors directly connected to a fuse asrecited in claim 6 further includes the step of using the transistorsdirectly connected to the fuse for sensing the presence or absence ofthe fuse.